Fixed binary incompatibility of netstandard2(0/1) build
Fixed binary incompatibility of netstandard2(0/1) build

The problem was in GetValueOrDefault method which was only added to netstandard20.
With this change, it is present in both builds. It's just not an extension method
in netstandard21

riganti/dotvvm,riganti/dotvvm,riganti/dotvvm,riganti/dotvvm